1. Field of the Invention
This invention relates to a burner apparatus and method for reducing nitrogen oxides, formed during combustion of gaseous fuel, by operating with a fuel-lean primary combustion chamber, staged fuel injection and internal flue gas recirculation within a secondary combustion zone, external flue gas recirculation from downstream of the secondary combustion zone back to the primary combustion zone, and/or bypass of a portion of primary gaseous fuel and excess oxidant from the primary combustion zone to the secondary combustion zone.
2. Description of Prior Art
Many conventional gas-fired burners use a diffusion flame combustion process in which combustion occurs over a range of equivalence ratios, including high temperature, lean regions where thermal nitrogen oxides (NO.sub.x) form. One known method for reducing peak flame temperatures is to use a combustion process which creates a fuel-rich primary combustion zone and subsequent air staging with corresponding heat loss, resulting in lowering the overall combustion equivalence ratio to achieve complete combustion.
Another known method for reducing peak flame temperatures relates to a combustion process that operates with a fuel-lean primary combustion zone and fuel staging in order to raise the equivalence ratio. However, such known methods of staged fuel combustion rely upon a diffusion flame to produce the lean primary stage. External flue gas recirculation has been added to such known methods for further reducing NO.sub.x.
In the combustion of gaseous fuels, NO.sub.x is formed primarily through fixation of molecular nitrogen and oxygen in the combustion air. It is known that thermal NO.sub.x formation depends on the existence of flame regions with relatively high temperatures and excess oxygen. Many conventional combustion methods for reducing NO.sub.x are based upon avoiding such conditions.
It is necessary to consider the prompt NO.sub.x formation process in order to reach very low NO.sub.x levels. Reactions between hydrocarbon fragments and molecular nitrogen can lead to the formation of bound nitrogen species, such as hydrogen cyanide (HCN), which can subsequently be oxidized to nitrogen monoxide (NO). Such process becomes significant relative to the thermal mechanism under moderately fuel-rich conditions at relatively lower temperatures. Avoiding such conditions can reduce prompt NO.sub.x contributions.
Faulkner, U.S. Pat. No. 5,275,554 discloses a combustion system for reducing NO.sub.x emissions by recirculating flue gas and a secondary fuel into a combustion chamber of a heat exchanger, adjacent an outlet end of a burner. A low NO.sub.x manifold housing is rigidly coupled between the heat exchanger and a conventional gas and oil burner. The '554 patent apparently teaches stoichiometric combustion within the burner.
Martin et al., U.S. Pat. No. 5,044,932 teaches a process and apparatus for reducing NO.sub.x content of flue gas effluent by internally recirculating flue gas into a primary combustion zone. Fluid driven eductors are used to enhance the amounts of collected internally recirculated flue gas into the primary combustion zone.
Schol, U.S. Pat. No. 3,838,652 discloses a burner apparatus which is used to recycle flue gas through openings arranged circumferentially in a duct member. The flue gas flowing through the openings and through a flame hole into a primary combustion chamber forms a cooling mantle of flue gas enveloping the flame of the burning fuel emanating from the burner.
There is an apparent need for a burner apparatus and method which operate with staged fuel combustion wherein a primary combustion zone operates under fuel-lean conditions and at a relatively low temperature, and in which both stages operate with overall fuel-lean stoichiometry.